1. Field of the Invention
The present invention relates to a magnetometer, and more particularly to a magnetometer using a Josephson device, which is well suited to the detection of a weak magnetic field.
2. Description of the Related Art:
In the technical field concerned, magnetometers using Josephson devices are known as superconducting quantum interference device (hereinafter, referred to as SQUID) magnetometers. The conventional magnetometers are represented by a DC-SQUID magnetometer and an rf-SQUID magnetometer. The DC-SQUID magnetometer is an apparatus wherein a magnetic flux interlinking with a superconducting loop which consists of two Josephson junctions and an inductor is observed in the form of direct current as the change of the maximum superconducting current which flows through the superconducting loop (refer to, for example, U.S. Pat. No. 4,389,612 and U.S. Pat. No. 4,567,438). The rf-SQUID magnetometer is an apparatus wherein a magnetic flux interlinking with a superconducting loop which consists of a single Josephson junction and an inductor is observed in the form of alternating current as the change of the maximum superconducting current which flows through the superconducting loop. In any of the conventional SQUID magnetometers, the SQUID forming the key point thereof is a passive device which senses the magnetic flux interlinking with the superconducting loop. The SQUID using the Josephson junction is immersed in liquid helium, and the output signal thereof is sent through a connecting cable to a measurement circuit as well as a data processor at the room temperature. The SQUID measures a very feeble magnetic flux, and the output signal of the Josephson device is also feeble in itself. Therefore, the signal which is sent from the SQUID to the measurement circuit at the room temperature is a very feeble signal of, for example, about 1 .mu.V. In consequence, the sensitivities of the conventional SQUID magnetometers have been limited by thermal noise at the room temperature.
Further, a magnetometer for detecting a weak magnetic field is discussed in "IEEE Trans. on Electron Devices," ED27, No. 10 (1980), pp. 1896-1908. In this magnetometer, a voltage signal from a magnetic field detecting device (for example, DC-SQUID) is amplified and is subjected to phase-sensitive detection by a lock-in amplifier, the detection output of which is fed back to the detecting device.
The above prior art does not take into consideration the influence of noise which is applied to a magnetometer circuit or the detecting device through the grounding wire of the circuit. In particular, there has been the problem that, when the magnetometer circuit is operated by an A.C. power source, the magnetometer operates erroneously due to noise from an A.C. power source line.
Magnetometers in each of which, in order to solve such a problem, the magnetic flux detecting device susceptible to noise and the peripheral circuit thereof are connected by photo-isolator means so as to electrically insulate them, are disclosed in the official gazettes of Japanese Patent Applications Laid-open No. 82872/1985, No. 35378/1986 and No. 77772/1986.
These prior-art apparatuses adopt the pulse width modulation (PWM) for converting an electric signal into light. However, they do not take into consideration an influence exerted on the magnetic flux detecting device by pulsed noise developing in a modulator portion of the PWM, that is, the problem of an erroneous operation.
Further, the prior-art magnetometers employ an optical fiber or the like as a feedback circuit and therefore have no problem on the rate of response in that portion. Since, however, electrical elements are respectively connected to an optoelectric converter and an electrooptic converter disposed on both the ends of the optical fiber, there is the problem that the merit of high-speed transfer of the optical fiber is not satisfactorily demonstrated due to the electrical elements.